Memory is constant and therefore throws an error when trying to assign to it

[CEbbinghaus]

typed-screeps/src/memory.ts

Line 15 in ae85cfd

declare const Memory: Memory;

Pretty self explanatory. Directly encouraged by the docs this is needed for allowing for custom serialization

[Jomik]

No, it actually doesn't. It is trying to explain to you how access to Memory works :)
Essentially, on first access in a tick, it will do a JSON parse of the raw memory and assign it to the variable.
If you want to do the "memory hack", you could do something like https://github.com/Jomik/screeps-ai/blob/27a5edce0ac7920f5b4feb04c3dd6808527d3509/packages/bot/src/utils/memory-hack.ts#L9

I suppose the proper keyword would be let, but I think it is better to be very explicit if you mess with Memory, as it can really screw you over.

[DocDvorak]

@Jomik is correct. Here is the Memory property definition. On first access, the getter is replaced with an accessor, which allows it to be assigned.